1. Field of the Invention
The invention relates to rotor balancing devices and is particularly although not exclusively applicable to balancing devices for motor vehicle propeller shafts.
2. Description of the Prior Art
U.K. Patent No. 832048 discloses an automtic balancing device comprising a housing having an annular cavity filled with a dampening fluid, for example oil or ethylene glycol. Within the dampening fluid, rolling bodies are arranged to be freely movable. Should an unbalance occur in the rotary body during operation thereof, the rolling bodies will roll around the annular cavity and will assume, under the action of the components of force set up by the unbalance, positions in which they compensate for the out of balance. The specification also discloses the use of a solid but low melting point damping medium which can render the balls substantially immovable when cold, for example during a starting period or during any other phase of operation. The damping medium may be melted either by friction occurring during operation of the rotating body or by external heating.
The disadvantage with the device having a damping fluid described above is that the rolling bodies will tend to move out of their balancing positions when the device is at rest so that the device will always be out of balance when it is started from rest and will not be in balance until it has been rotated above its resonant frequency speed.
The disadvantage with the device having a low melting point damping medium described above is that heating of the damping medium causing it to melt and allowing the rolling bodies to move will occur in use of the device so that when the bodies can move out of the balancing positions if the device is not rotating just above its resonant frequency speed and the bodies could then act to increase the out of balance of the device.
It is an object therefore of the present invention to provide a method and means for balancing a rotor in which the rotor, once balanced, remains permanently balanced.
The applicants co-pending U.S. Pat. Appliction Ser. No. 625242, pending in Group 230 discloses a rotor balancing arrangement in which a rotor to be balanced has an annular cavity containing two balls free to roll around the cavity. An initially inert heat sensitive adhesive is coated around the cavity and the rotor is rotated to a speed just have its first resonant frequency vibration so that the balls take up balancing positions in the cavity counter-balancing the out-of-balance of the shaft. The adhesive is then activated by an external heat source to lock the balls in the balancing position so that the shaft is permanently balanced. In the latter arrangement balls are required which are free to rotate around the inner surface of the annular cavity until they reach the balancing position and it is an object of the present invention to provide an arrangement which avoids the rolling of weights around the annular cavity.